The alignment of satellite galaxies in gasdynamic simulations

卫星被限制在星系的极地地区，这一问题困扰了天文学家至少40年。在这中间的四十年里，无论是在外部星系（Holmberg于1969年首次发现）还是在我们自己的银河系中观测到的这种效应，都没有得到充分的解释。事实上，最近在SDSS和2dFGRS等大规模调查中对卫星星系排列的观察与所谓的霍尔姆伯格效应相矛盾，并发现卫星倾向于与宿主星系的平面对齐。不管各向异性分布的意义是什么，卫星在它们的母晕内并不均匀分布，这使得许多作者强调，这与标准协和宇宙学的简单预测是不一致的。到目前为止，理论描述试图将模拟中发现的卫星各向同性分布与观测结果相一致。许多作者只使用暗物质模拟来研究本星系群中卫星的排列。尽管在理解卫星为何呈各向异性分布方面取得了一些进展，但它们相对于中心星系的扁平化却几乎没有得到分析。原因很简单：在大的宇宙体积中进行流体动力学模拟是昂贵的，而且对于确定中央星系盘的方向是必要的。通过使用有史以来最大的流体动力学模拟之一，我们希望解决一个40年来未解之谜：银河系的极对是否是个严重的例外？或者它的变平是类似星系的典型现象。

The confinement of satellites to the polar regions of galaxies has confounded astronomers for at least 40 years. Neither the observations of this effect in external galaxies (first seen by Holmberg 1969) nor in our own Milky Way have been adequately explained in the intervening four decades. In fact, more recent observations of the alignment of satellite galaxies in large scale surveys such as the SDSS and in the 2dFGRS have contradicted the so-called Holmberg effect and found that satellites tend to align themselves with the plane of their host galaxies. Regardless of what the sense of the anisotropic distribution is, that satellites are not uniformly distributed within their parent halos has caused a number of authors to highlight that this is in disagreement with the simple predictions of the standard concordance cosmology. Until now, theoretical descriptions have tried to reconcile the isotropic distributions of satellites found in simulations with the observations. Many authors have employed dark matter only simulations in a bid to study the alignment of satellites in the local group. Although some progress has been made in understanding why satellites are anisotropically distributed, the sense of their flattening with respect to the central galaxy has all but evaded analysis. The reason for this is simple: running hydrodynamic simulations in a large cosmological volume is expensive and is necessary for determining the orientation of a central galactic disc. By using one of the largest hydrodynamical simulation ever run we wish to address a 40 year old unsolved mystery: is the Milky Way a gross exception with its polar alignment? or is its flattening typical of similar galaxies.